The purpose of this project is to determine how children with Down Syndrome and severe behavioral disturbance (autism, stereotypes, self- injurious behavior) differ from their Down Syndrome peers without severe behavioral disturbance. Quantitative image analysis performed on brain MRIs is being used to determine if the DS/SBD subjects show a greater degree of dysgenesis in the frontal cortex and cerebellum. Neuropsychological and behavioral testing are being performed to determine the age of onset, topography, frequency, intensity and progression of behavioral symptoms. Genetic testing is being performed to determine the specific cytogenetic abnormality (trisomy, translocation or mosaicism) responsible for the Down Syndrome.